In cases where an oil pressure path or a spool hole is provided inside a main body of an oil pressure apparatus, a certain method has been well-used. in this method, an oil pressure circuit hole is provided so as to be opened with respect to an outer surface of the main body, and then, an aperture end portion of this oil pressure circuit hole is blocked with a plug. The plug herein includes a shaft portion and a head portion. An external thread portion is disposed on a leading end of the shaft portion, while the head portion is disposed on a base end portion of the shaft portion. The head portion herein is a portion on which a tightening tool such as a wrench is mounted. The head portion is further shaped like a polygonal column which has an external diameter larger than that of the shaft portion. A storage groove is provided to the base end portion of the shaft portion in order to mount an O-ring. The storage groove herein is a ring-shaped recess. An external diameter of the storage groove is configured to be smaller than that of the shaft portion.
In a case of applying this plug and blocking the aperture end portion of the oil pressure circuit hole, the plug is screwed together with the aperture end portion of the oil pressure circuit hole with the O-ring mounted on the storage groove. In cases where the plug is screwed together with the oil pressure circuit hole with a preset standard torque, the O-ring compressed between the plug and an inner periphery of the oil pressure circuit hole is pressed by an outer periphery of the storage groove and the inner periphery of the oil pressure circuit hole. As a result, the oil pressure circuit hole and the plug are sealed (see, for example, Patent Literatures 1 and 2).
In order to prevent oil leakage from between the oil pressure circuit hole and the plug, it is necessary to maintain a state in which the plug is screwed together with the oil pressure circuit hole without a backlash. In other words, when axial force continuously acts on the plug, it is possible to maintain the state in which the O-ring is pressed by the outer periphery of the storage groove and the inner periphery of the oil pressure circuit hole, and to prevent the oil leakage from between the oil pressure circuit hole and the plug. The axial force acting on the plug is reaction force of elastic deformation in a longitudinal direction. The reaction force applied to the plug is caused when the plug is screwed together with the oil pressure circuit hole. In a case of the above-mentioned plug, the external diameter of the storage groove is configured to be smaller than that of the other shaft portion. Therefore, the axial force acts on the plug because a part where the storage groove is disposed elastically deforms in the longitudinal direction when the plug is screwed together with the oil pressure circuit hole.
However, in the shaft portion, regarding an amount of the elastic deformation of the part where the storage groove is disposed, it can be expected to have only a small amount. Accordingly, even. in a case where little plastic deformation or looseness appears on the head portion of the plug which is to be a bearing surface, for example, due to an influence of aging, the axial force disperses and the plug gets loose. As a result, there is a possibility that oil may leak from a gap between the plug and the inner periphery of the oil pressure circuit hole. Specifically, the above-mentioned problem is further obvious in a case of a plug disposed inside a main body of an oil pressure apparatus to block an oil pressure path or a spool hole. It is because extremely high oil pressure of about 480 standard atmospheres repetitively acts on the plug.
On the other hand, Patent Literature 3 discloses a plug for blocking an oil filler of an operating oil tank. The plug herein is provided for blocking the oil filler while involving a washer between the plug and an aperture end portion of the oil filler.